Display modules are used in portable devices to display information in graphical form. Small size is an important aspect in portable devices. However, the small size of a portable device also sets a limitation to the size of a display incorporated in said device. A typical drawback of a conventional small display is that an observer can examine only a small portion of a large displayed image at a glance, while preserving adequate resolution.
Large images may be displayed by a small device e.g. when the device comprises a near-eye virtual display. An imaging optics may convert a small real image generated by a micro-display into a virtual image. An observer may place the device near his eye such that when light provided by the imaging optics impinges on his eye, he perceives an impression of a large detailed virtual image displayed at an infinite distance.
The micro-display and the imaging optics may be made even smaller and/or lightweight when the light beam provided by the imaging optics is expanded by using a diffractive beam expander, which is also known as an exit pupil expander (EPE). A near-eye display based on a diffractive beam expander is disclosed e.g. in a patent application EP0535402. U.S. Pat. No. 6,580,529 discloses a diffractive beam expander for expanding a light beam in two dimensions, i.e. horizontally and vertically.
US 2006/0126182 discloses a diffractive beam expander comprising a first diffractive element to couple light into a substrate, a second diffractive element to couple light out of the substrate, and an intermediate diffractive element to diffract in-coupled light towards the second diffractive element. The intermediate diffractive element has a substantially periodic pattern composed of substantially linear elements for producing conical diffraction.